The most common characteristics of a Romantic hero are that he must be strong-willed, charismatic, and handsome, the sort of man that people are drawn to. But not only that, a Romantic hero must also feel alienated. Victor Hugo, for the most part, presents Marius as a classic Romantic hero who matches these characteristics.
We certainly do see Marius as a strong-willed, charismatic, handsome figure. We see him being portrayed as strong-willed when he decides to tell his grandfather he will not stop seeing his father and is thereby disinherited by his grandfather. While Marius used to be wealthy due to his grandfather's provisions, he must now suffer poverty. His poverty even leads to his feelings of isolation. As Hugo describes, Marius was very handsome, with thick, black hair, and women were attracted to him; but anytime Marius saw women looking at him, he felt that they were looking at his shabby clothes and therefore isolated himself. As Hugo explains:
He had observed that young girls turned round when he passed by, and he fled and hid ... He thought they were staring at him because of his old clothes ... This mute misunderstanding between him and the pretty passers-by had made him shy. (Vol. 3, Bk. 6, Ch. 1)
We even see him acting with charisma when he goes to join the fighting in the barricades, wishing to die because Cosette has been taken from him. He particularly acts charismatically when he bravely clears the barricade of all enemies by threatening to blow the barricade up with a keg of gun powder. His initiative and bravery makes Enjolras declare Marius to be the chief of the barricade.